I will be arriving in Berlin around 1 pm, and staying for 2 nights. Which card is best to get, or are they even worth it ?
Depends on what you want to do. Do you just want to use public transportation, are you looking to go to a lot of museums, looking for bus tours, or what. Read what each card offers and see how well it matches what you want to do in Berlin.
Both are combined public transport tickets and discount booklets. The difference between them is the composition of discount partners.
As Jo said, compare the lists of partners (City Tour and Welcome (pdf)) with the things you want to do.
If you just use a few dicounts it may even be cheaper to use just normal (24h or 7-day) transit tickets and pay regular entrance fees.